I am going to install a greywater system from my shower. I am wondering if a branched drain is necessary? Can I utilize the type of distribution that greywater action teaches for the L2L system (with 1″HDPE tubing than somewhat “branches” out as it takes water to each corresponding mulch basin)? Or do I need to use an actual “branched drain” that is 1.5″ to 2″ of ABS tubing with flow splitters?

I will not be distributing the water far from the source (not more than 25′) and the slope is significant (I haven’t measured yet but I bet it is at least 2%).

For gravity flow systems, like from a shower, it is best to use rigid pipe (ABS) because you can maintain slope the entire way and ensure there aren’t low points, which are a clogging potential. (Solids in the water, like hair, settle out any time the water flow slows down too much.)

That said, in some situations it may be okay to use 1″ HDPE but you’d have to be okay with potential clogging and needing to unclog the system. It will be a system that you need to stay involved in for it to work well! The 1 x 1/2″ barbed tees that are used in an L2L system won’t work the same for a shower because there isn’t the pressure of the pump pushing water through. If you really want to use HDPE tubing I wouldn’t plan on having more than a couple outlets (and you’ll probably need to use just 1″ tees, or maybe 1 x 3/4″ tees). I’ve only experimented a little with this kind of system so can’t offer much more advice. Also, your design will be affected by how much greywater is produced and how many plants you’re trying to irrigate.